Leading biological based pesticide utilizes Bacillus thuringiensis (abbreviated herein as Bt) against lepidopterans, coleopterans, and other insect pests. The Gram-positive spore-forming bacterium produces parasporal protein crystals during stationary phase of the growth cycle. Genes encoding the crystals are categorized as cry genes. As a pesticide, the primary mode of action involves protein solubilization, proteolytic activation of the protoxin, binding to epithelial midgut receptor (cadherin), and subsequent pore formation and/or activation of intracellular cell-death signaling pathway. The use of Bt and its effectiveness as an insecticide is largely dependent on receptors in the target insect and the solubility of the Cry protein.
Coleopteran pests cause extensive damage to crops in the United States. For example, damage to corn crops occurs when rootworms feed on corn seedling roots. It has been estimated that rootworms cause in excess 1 billion dollars in damage to corn crops in the United States. (Meycalf, R. L., et al., 1986. Drysan, J. L. and T. A. Miller [Eds.], Springer-Verlag, New York, N.Y., pp. vii-xv.) Even with chemical insecticide applications of organophosphate or pyrethroid, rootworm damage still causes an estimated $750 million dollars annual damage to corn crops. One approach to combat rootworm damage while decreasing dependence of chemical pesticides is to express Bt protein in transgenic corn. For instance, Bt strain PS149B1 confers resistance to rootworms in corn plants (Moellenbeck, et al., 2001. Nature Biotechnology, 19:668-672). Additionally, U.S. Pat. Nos. 4,797,276 and 4,853,331 disclose a Bacillus thuringiensis strain san diego, (NRRL B-15939) that is effective in controlling corn rootworm, among other coleopteran species. Given the widespread damage caused by rootworm, there is continuing need to develop efficient biologically-based insecticides, specifically potentiating the usage of Bt toxin.
A lepidopteran pest is the pink bollworm (Pectinophora gossypiella). It is estimated that the preventive cost, damage control, and crop lost costs cotton growers over thirty-two million dollars annually (National Cotton Council, 2004). Cotton crop damage occurs when female bollworms lay their eggs in cotton bolls during the summer mating season. Resulting larvae feed on cotton seeds upon chewing and burrowing through cotton lint. Techniques to combat pink bollworm include releasing pheromones to disruption mating, releasing sterile males to disrupt mating patterns, chemical insecticide treatments, and planting of transgenic Bt cotton.
While Bt pesticides have been used against a narrow range of lepidopteran pests, the discovery that Bt can have a broader application towards other Orders of insects has prompted its use targeting other pests. (For instance, see U.S. Pat. Nos. 4,797,276 and 4,853,331). Also, it has been reported that the expression of Cry3Aa in transgenic potato cultivars are resistant to Leptinotarsa decemlineata while exerting a deleterious effect on the polyphagous moth Spodoptera littoralis (Hussein et al., 2006. Journal of Chemical Ecology, 32:1-13). Novel Bt isolates, new uses of known Bt isolates, and potentiating the toxicity of existing toxins remains an empirical, unpredictable art.
One approach to increase potency of Cry proteins against various insect pests is to utilize a Bt toxin receptor to potentiate toxicity to a target pest. The approach uses a peptide fragment derived from an insect cadherin protein combined with Cry protein toxin to increase a synergistic potency that would not be achieved via administration of the partial cadherin fragment or Cry protein individually.